1. Field of the Invention
An object of the present invention is a method to apply pulse gradients in an imaging experiment by means of a nuclear magnetic resonance (NMR) machine. The invention can be applied especially in the medical field where NMR imaging techniques are used as diagnostic aids. However, it can also be used in other fields, especially in industry, for non-destructive testing.
2. Description of the Prior Art
The phenomenon of nuclear magnetic resonance and the working principle of imaging machines, based on this phenomenon, are known. The NMR signal depends at each position in a field under examination by an imaging machine of this type on the orienting field of this imaging machine. To bring out the phenomenon, a body to be examined is excited with a radio-frequency excitation. A free precession decay signal from the body's particles, when they return to equilibrium, can be measured when their magnetic moment is re-oriented with the orienting field at the end of their flip which depends on the excitation. The excitation is applied to the entire body under examination and all the particles of the body may give out a free precession decay signal at its end. In fact, the rise of this phenomenon is distributed among selected sections of the body by local modification of the conditions of resonance. This modification is given by an additional magnetic encoding known as selection encoding.
The local resonance conditions are modified by increasing or decreasing the intensity of the orienting field in the selected section. For scientific and technological reasons, it is not possible to modify the intensity of a continuous magnetic field sharply on either side of a section of space. The modification of the field thus takes the form of a regular variation according to the abscissa of the positions of the space considered on an axis perpendicular to the section to be selected. Hence, the intensity of the orienting field is subjected, along this axis, to a gradient. In practice, the additional fields, added to the constant and homogenous fields throughout the space in order to provoke this variation are commonly called field gradients.
To create an image of a selected section, the free precession signal resulting from the excitation must also be subjected to other additional encodings before and during its measurement. The additional encodings applied before the measurement are called phase encodings. In a series of successive sequences, the encoding thus done is modified from one sequence to another. The resulting variations in the measured signal are interpreted to discriminate the image parameter as if they were projected on an axis parallel to the one along which these phase encodings are applied. When the de-excitation signal is being read, another encoding is further applied in the form of a so-called read gradient. This encoding tends to modify the resonance frequency of the resonance phenomenon according to the amplitude of the additional magnetic field thus provided. The result of this is that it is also possible to do a frequency discrimination in the signal received showing, at each sequence, the result of the projection of the image on another axis, parallel to the read gradient axis. By sampling the read signal at each sequence in the series, a set of samples forming a matrix is obtained. Calculating the Fourier transform along the rows and then the columns (or conversely) of the samples matrix gives the image. This image corresponds to that of the section selected by application of the selection gradient. This imaging mode requires the gradients to be pulsated: they are applied by pulse gradients.
Conventionally, magnetic coils to apply pulse gradients are set up on the machines so as cause the field gradients to be applied such that they are oriented fixedly with respect to the machine. These orientations are generally distributed along three mutually perpendicular axes, one of which is parallel to the orienting field (z) of the machine. If X, Y and Z are the imaging axes of the machine, the selection, phase encoding and read functions can be assigned to each of these three axes respectively to define transversal, sagital and frontal images of the body. More generally even the making of oblique images has been envisaged. In theory, making these images does not raise any special difficulties.
In practice, it is not so simple to make oblique images. For the selection, phase encoding and read gradient pulses have their own forms which cannot be transposed (to change the function of the machine's imaging axes) but which are difficult to associate owing to the very fact that there is a variety of forms and variation in encoding. In particular, as stated above, the phase encoding changes from one sequence to another in the series. Thus the combination, on an imaging axis of the machine, of a phase encoding pulse with a read pulse and/or a selection pulse amounts to defining complex forms of gradient pulses. This is all the ruer as the phase encoding pulses are variable in time. It becomes necessary, for example, to make pulses having a broadly staircase-like form with at least two steps where the height of one step varies from one sequence to another. It is difficult to programme power supplies of the gradient coils for these tasks. The result of this is that practitioners do not use oblique images, and this is detrimental to the quality of their diagnosis.